The invention is directed to a method of forming deep, highly anisotropic pores for microchannel plates (MCP). In particular, the present invention describes a fabrication method for a micromachined MCP, which addresses various practical limitations in current techniques, and offers substantial added flexibility for the construction of a bulk semiconducting MCP. This disclosure also details an ultra-compact, microfabricated image tube and photomultiplier tube which incorporate micromachined MCPs of any construction.
The invention is related to the method for making microchannel plates described in U.S. Pat. Nos. 5,086,248 (1992) and 5,205,902 (1993) by J. R. Horton and G. W. Tasker, assigned to Galileo Electro-Optics Corporation, the assignee herein. The dynode structure and activation methods used for the invention are described in U.S. Pat. No. 5,378,960 by G. W. Tasker and J. R. Horton, assigned to Galileo Electro-Optics Corporation, the assignee herein. The teachings of the above-identified patents are incorporated herein by reference.
The basic methodology of the process described in U.S. Pat. No. 5,205,902 employs a directionally applied flux of reactive particles against at least one face of a substrate wafer to produce a series of highly anisotropic, high aspect ratio, parallel microchannels which are then activated with thin films to make an electron multiplier. The applied flux of reactive particles is produced by one of several methods of reactive plasma etching (RPE). As defined herein, the aspect ratio (.varies.) refers to the depth of a pore (1) divided by its width (d). (1 and d are conventional terms known to those skilled in the art, and are related to the terms length and diameter in circular channel electron multipliers.)
During RPE of pores, the etch rate of the substrate material tends to decrease with increasing .varies.. This effect is enhanced for pore diameters less than roughly 5 .mu.m and thus slows RPE of pores with diameters and aspect ratios suitable for microchannel plates. This reduction in etch rate may be the result of many different physical processes and is discussed in the literature. One physical process thought to produce this effect is a transport limitation of the reaction products of the etch out of the pore beyond some threshold aspect ratio and is generally referred to a microloading. The process is analogous to molecular flow conductance limits in vacuum systems. Another physical process which may decrease the etch rate with increasing aspect ratio is the deflection of reactive ions away from the bottom of the pore into the sidewall due to electric field effects at the pore opening. This is generally referred to as reactive ion etching (RIE) lag. Both of these effects, as well as other related effects, makes directly etching high aspect ratio, small pore with d of approximately 1 .mu.m difficult and time consuming.
As described in U.S. Pat. No. 5,205,902, the substrate used for the microchannel electron multiplier must be masked with a photo-patterned, etch resistant material. Very often, this mask material is dielectric, such as an oxide or nitride, since these materials have slow etch rates in RPE compared to the semiconductor materials used for the MCP substrate in one embodiment. However, an alternate embodiment specifies a dielectric material for the MCP substrate. Since the etch rate of the dielectric substrate is slow, the selection of suitable etch masks is limited for forming deep high aspect ratio channels. The slow etch rate can be addressed in part by newly available high-density plasma etch techniques such as inductively coupled plasma (ICP) etching and electron cyclotron resonance (ECR) etching, however, the selectively of the mask to the substrate is still an issue.
Relatively thick metals (e.g., .gtoreq.1 um) can be used as etch masks for these dielectrics. However, patterning thick metal is difficult and time consuming. Together these difficulties limit the practical manufacturability and commercial viability of micromachined MCPs constructed by direct etching of pores in dielectric materials.